Is it possible to start and restart the Bitcoin Core service during the syncing process without loss of work? What happens to syncing if the Server fails or shuts down, i.e. a power failure? I would like to put it on a UPS but that requires stopping and restarting the BTC Core service. I am running the service on an Embassy One server
Bitcoin Core saves the current chain state to disk, so you can safely shut it down and it will continue where it left off after you start it again. Power failures are a bit more complicated because your disk can become corrupted, and I can find several posts from people experiencing problems with Bitcoin Core after their power went out, so I wouldn't assume it's safe against power failures.